Method for the separation of carbon isotopes by chemical exchange method

ABSTRACT

A method for the separation of the isotopes of carbon comprising contacting an aqueous solution containing an acid-dissociated type chemical species of a carbon-containing acid and a non-dissociated type chemical species of said acid with an anion exchange resin as a solid phase, thereby to allow an isotope exchange reaction with respect to carbon to proceed between said acid-dissociated type chemical species and said non-dissociated type chemical species and, concurrently, to adsorb said acid-dissociated type chemical species on said anion exchange resin, and separate C 12  and C 13  respectively into a solution phase and the solid phase or the solid phase and a solution phase, whereby C 13  is concentrated. This method can be practised at an extremely favorable efficiency and inexpensive cost, as compared with the prior art method for the separation utilizing a distillation method or a chemical exchange method between gas--liquid phases, gas--solution phases, solution--solution phases and the like.

FIELD OF THE INVENTION

This invention relates to a new method for the separation of carbonisotopes. More particularly, it is concerned with a method for theseparation of the isotopes of carbon utilizing an isotope exchangereaction with respect to carbon between a dissociated type chemicalspecies of a carbon-containing acid and a non-dissociated type chemicalspecies of said acid wherein the desired carbon isotope is separated andconcentrated in one of the below-mentioned phases by the separationbetween a solid phase and a solution phase using an anion exchangeresin.

DESCRIPTION OF THE PRIOR ART

Naturally existing carbon is a mixture of about 98.89% by weight ofcarbon possessing a mass number of 12 (C¹²) with about 1.11% of carbonpossessing a mass number of 13 (C¹³).

C¹³ has been employed for labeling of chemical compounds in variousfileds such as clinical medicine, pharmaceutics, biochemistry,agriculture or the like by using its nonradioactivity and, hence, theneed for C¹³ has been highly increased. Consequently, the development ofa technique for more efficient separation of C¹³ at a higherconcentration has been desired in various fields.

The main methods for the separation of C¹³ presently utilized comprise acondensing distillation method, a thermal diffusion method, a chemicalexchange method and a laser method. Even the condensing distillationmethod, which has been at present regarded as the leading one amongthose methods, requires a considerably expensive cost for theseparation, which leads to a hindrance in the availability of C¹³.

The chemical exchange method, on the other hand, utilizes an isotopeexchange reaction with respect to carbon between differentcarbon-containing chemical species. More specifically, an isotopeexchange reaction with respect to carbon as represented by the followingchemical equation (1) can occur between different carbon-containingchemical species, CX and CY, and, in a case where its equilibriumconstant deviates from 1, contact of both chemical species can provideconcentration of C¹³ in one of both chemical species. ##STR1##

There have been hitherto known various combinations of chemical speciescapable of producing such an isotope effect.

However, all previous chemical exchange methods have been limited tothose utilizing an isotope exchange reaction between a gas phase and aliquid phase, between a gas phase and a solution phase, or between asolution phase and a solution phase. In the case where an isotopeexchange reaction between a gas phase and a liquid phase or between agas phase and a solution phase is to be utilized, the reactionefficiency becomes significantly lower because of exchange performedsolely on a gaseous surface. Also, in the case of an isotope exchangereaction between two solution phases, separation of different chemicalspecies is troublesome and the process for multiplying isotope exchangereactions tends to be complicated in order to attain the desired isotopeat a highly concentrated level. Therefore, the current chemical exchangemethods have not improved on the condensing distillation method. On theother hand, the condensing distillation method basically involves majorproblems in an economical aspect owing to a heavy energy consumption.

DISCLOSURE OF THE PRESENT INVENTION

This invention provides a method for the separation of carbon isotopesby a chemical exchange method, which may concentrate C¹³ with a higherseparation efficiency in comparison with the prior art methods. Moreparticularly, the invention employs an aqueous solution containing anacid-dissociated type chemical species and a non-dissociated typechemical species of a carbon-containing acid as differentcarbon-containing chemical species, contacts said aqueous solution withan anion exchange resin as a solid phase, thereby effecting an isotopeexchange reaction between said acid-dissociated chemical species andsaid non-dissociated chemical species. Concurrently saidacid-dissociated chemical species is adsorbed on said anion exchangeresin and the carbon isotope is separated into one of the phases.

The acid-dissociated type chemical species exhibits a strongadsorptivity to the anion exchange resin because of the chemical specieshaving a negative charge, while the non-dissociated type chemicalspecies is difficulty adsorbed onto the anion exchange resin, whereuponit is practicable to separate both chemical species preferably.

The term "a solid phase" as used herein is meant to include anacid-dissociated type chemical species having been adsorbed on the anionexchange resin as a solid phase.

In the case where a carbon-containing acid is represented as CXH and itsacid-dissociated type chemical species as CX⁻, an isotope exchangereaction with respect to carbon between an acid-dissociated typechemical species and a non-dissociated type chemical species may berepresented by the following chemical equation. ##STR2## (wherein Krepresents an equilibrium constant for isotope exchange)

Therefore, it is possible to separate C¹³ into the anion exchange resin(a solid phase) if K is more than 1 and into a solution phase if K isless than 1.

Accordingly, there is provided in the basic embodiment of this inventiona method for the separation of the isotopes of carbon, C¹² and C¹³,comprising effecting an isotope exchange reaction with respect to carbonbetween a dissociated type chemical species of a carbon-containing acidand a non-dissociated type chemical species of said acid, characterizedin that an aqueous solution containing an acid-dissociated type chemicalspecies of a carbon-containing acid and a non-dissociated type chemicalspecies of said acid is contacted with an anion exchange resin as asolid phase to adsorb the dissociated type chemical species onto theanion exchange resin, while allowing an isotope exchange reaction withrespect to carbon to proceed between the dissociated type chemicalspecies and the non-dissociated type chemical species, therebyseparating C¹² and C¹³, respectively, into a solution phase and thesolid phase or the solid phase and a solution phase, whereby C¹³ isconcentrated in the phase into which C¹³ is to be separated.

If isotopes are to be separated between a solid phase and a solutionphase and the non-dissociated type chemical species coexists in a solidphase and the acid-dissociated type chemical species in a solutionphase, it is believed that an apparent equilibrium constant for isotopeexchange reaction may be less than K. Then, if a total concentration ofthe carbon-containing chemical species in a solid phase is defined as Cand a total concentration of the carbon-containing chemical species in asolution phase as C, the isotope exchange reaction in this instance maybe represented by the following chemical equation wherein K' is apractically effective equilibrium constant for isotope exchange betweena solid phase and a solution phase ##STR3##

The present inventors have found that ratios of the acid-dissociatedtype chemical species and the non-dissociated type chemical speciespresent in a solution phase may be determined by the pH of the solutionphase, while ratios of both chemical species present in a solid phaseare also dependent upon pH of the solution phase. Hence, the preferredK' for isotope separation, defined as 0.3<K'/K<1, can be maintained ifpH is adjusted to a range meeting the following condition with regard topKa of the carbon-containing acid employed (-log Ka when an aciddissociation constant is defined as Ka). ##EQU1##

The more preferable condition for pH is to satisfy the followingequation, ##EQU2## Under the latter condition, K'/K is shown by theformula 0.7<K'/K<1.

The anion exchange resins which may be employed in this invention mayinclude the below-mentioned ones.

Namely, there may be utilized a non-crosslinking polymerizable monomer astyrene derivative such as styrene, methylstyrene, dimethylstyrene, 3,4, 6-trimethylstyrene, methoxystyrene, bromostyrene, cyanostyrene,fluorostyrene, dichlorostyrene, N,N-dimethylaminostyrene, nitrostyrene,chloromethylstyrene, trifluorostyrene, trifluoromethylstyrene,aminostyrene and the like, a vinyl sulfide derivative such asmethylvinyl sulfide, phenylvinyl sulfide and the like, an acrylonitrilederivative such as acrylonitrile, methacrylonitrile,α-acetoxyacrylonitrile and the like, a vinyl ketone such as methyl vinylketone, ethyl isopropyl ketone and the like, a vinylidene compound suchas vinylidene chloride, vinylidene bromide, vinylidene cyanide and thelike, an acrylamide derivative such as acrylamide, methacrylamide,N-butoxymethylacrylamide, N-phenylacrylamide, diacetone acrylamide,N,N-dimethylaminoethyl acrylamide and the like, N-vinylsuccinimide,N-vinylpyrrolidone, N-vinylphthalimide, N-vinylcarbazole, vinylfuran,2-vinylbenzofuran, vinylthiophenone, vinylimidazole,methylvinylimidazole, vinylpyrazole, vinyloxazolidone, vinylthiazole,vinyltetrazole, vinylpyridine, methylvinylpyridine,2,4-dimethyl-6-vinyltriazine, vinylquinoline, epoxybutadiene and thelike.

As a cross-linking polymerizable monomer, there may be utilizeddivinylbenzene, divinyltoluene, divinylxylene, divinylnaphthalene,divinylethylbenzene, divinylphenanthrene, trivinylbenzene,divinyldiphenyl, divinyldiphenylmethane, divinylbenzyl, divinylphenylether, divinyldiphenyl sulfide, divinyldiphenylamine, divinylsulfone,divinyl ketone, divinylfuran, divinylpyridine, divinylquinoline, diallylphthalate, diallyl maleate, diallyl fumarate, diallyl succinate, diallyloxatate, diallyl adipate, diallylamine, triallylamine, N,N'-ethylenediacrylamide, N,N'-methylene diacrylamide, N,N'-methylenedimethacrylamide, ethylene glycol dimethacrylate, triethylene glycoldimethacrylate, polyethylene glycol dimethacrylate and the like.

As the preferred resin composition which may be employed in thisinvention, there may be utilized a product obtained by chloromethylationand subsequent amination of a cross-linked high molecular compound,which is synthesized by addition copolymerization of styrene,vinyltoluene, ethylvinylbenzene and the like with divinylbenzene as maincomponents, a product obtained by amination of an addition copolymercomprising as main components a monomer with a functional group such aschloromethylstyrene, epoxybutadiene, acrylamide and the like and across-linking monomer such as divinylbenzene, triallylisocyanurate andthe like, a product comprising as main components a monomer with anitrogen atom capable of forming an exchange group such asN-vinylphthalamine, vinylimidazole, vinylpyridine, vinyltetrazole,vinylquinoline, divinylpyridine and the like and, if necessary,copolymerized with a cross-linking polymerizable monomer.

As the amine which may be employed for the amination, there may beutilized an aliphatic amine such as triethanolamine, triethylamine,trimethylamine, triallylamine, diethanolamine, diallylamine,diethylamine, dimethylamine, 2-aminoethanol, ethylamine, methylamine,ethanolamine and the like, an aromatic amine such as aniline,o-aminophenol, N,N-dimethylaniline, N-methylaniline, m-toluidine,p-toluidine, p-aminophenol, diphenylamine and the like, a heterocyclicamine such as pyridine, γ-picoline, piperidine, pyrazine, piperazine,indoline, indole, imidazole, 2-methylimidazole, quinoline, 2,6-lutidine,1, 2, 3, 4-tetrahydroquinoline, N-methylpyrrolidine, benzotriazole andthe like.

For the preparation of the anion exchange resin employable in thepresent method from the above-recited respective components, there maybe adopted any methods commonly used in the art, but they will be moreillustratively explained hereinbelow.

A polymerization reaction is carried out by adding a polymerizationinitiator to a mixture of the non-crosslinking polymerizable monomer at6 to 98% by weight, preferably 10 to 90% by weight, more preferably 20to 80% by weight with the crosslinking polymerizable monomer at 2 to 94%by weight, with regard to a total weight of said non-crosslinkingpolymerizable monomer and said cross-linking polymerizable monomer.

The polymerization initiator may include a peroxide such as benzoylperoxide, methyl ethyl ketone peroxide and the like, an azo compoundsuch as azobisisobutyronitrile, 2-cyano-2-propylazoformamide and thelike. The amount of the polymerization initiator to be added is 0.01 to12% by weight, preferably 0.1 to 5% by weight, more preferably 0.2 to 3%by weight, based on the total weight of said monomers.

A polymerization temperature may be usually in the range of 0° C. to200° C., preferably 15° C. to 160° C., more preferably 30° C. to 130° C.

A polymerization time may be usually in the range of 30 minutes to 50hours, preferably 1 hour to 30 hours, more preferably 2 hours to 20hours.

After completion of the polymerization, the product is cooled, washedwith methanol and a large excess amount of water and then dried.Thereafter, chloromethylation is effected by introducing the productinto chloromethyl methyl ether. A reaction temperature for thechloromethylation is usually 2° C. to 10° C., while a reaction time is 1hour to 80 hours, preferably 10 hours to 60 hours.

In the amination, the polymer prepared as stated above is subjected toan amination reaction in a 5% to 40% ethanolic solution of theparticular amine to be used for the amination. An amination reactiontemperature is usually 20° C. to 80° C. and a reaction time is usually10 minutes to 10 hours.

The anion exchange resin which may be employed in the present method maybe preferably in the dissociated state with respect to a large portionthereof under the pH condition of a solution phase. The presentinventors have found that when pKa_(R) of the anion exchange resin (-logKa_(R) where an acid dissociation constant of said resin is defined asKa_(R)) is taken, the following condition should be essential between pHof a solution phase and said anion exchange resin in order to dissociatenot less than 99% of said resin.

    pKa.sub.R >pH+2

In fact, the _(p) Ka_(R) value for said anion exchange resin can bedetermined as shown below. First, 10 g (dry weight basis) of the anionexchange resin to be employed is converted to its Cl⁻ adsorption form bythe coexistence of 100 cc of a 0.1 M/l aqueous solution of hydrochloricacid. A 0.1 M/l aqueous solution of sodium hydroxide is added dropwisethereto with 0.1 cc portions and the pH of each of the resultantsolutions is measured. The pH value at an inflextion point in thethereby obtained titration curve is defined as pKa_(R) for the anionexchange resin, on condition that pKa_(R) value of a strongly basicanion exchange resin having a quaternary amine as an exchange group isdefined as over 14.

As the carbon-containing acid to be employed, there may be utilized anorganic acid such as formic acid, acetic acid, lactic acid, butyricacid, oxalic acid, citric acid, succinic acid and the like and aninorganic acid such as carbonic acid, hydrogen cyanide, cyanic acid,thiocyanic acid and the like.

In view of the requirement that an equilibrium constant for isotopeexchange should be greater between an acid-dissociated type chemicalspecies and a non-dissociated type chemical species, carbonic acid,hydrogen cyanide, cyanic acid, thiocyanic acid and formic acid arepreferable among these carbon-containing acids, with carbonic acid beingparticularly preferable.

The concentration of the acid to be employed is preferably 0.01 to 0.9M/l.

The instance wherein carbonic acid is used as a carbon-containing acidwill be illustrated below.

In case that carbonic acid is used, a bicarbonate ion (HCO₃ ⁻) and adissolved CO₂ may be applied as an acid-dissociated type chemicalspecies and a non-dissociated type chemical species, respectivey. Sincethe primary acid dissociation constant for carbonic acid is 10⁻⁶.3 at25° C. (pKa=6.3), the condition can be provided wherein CO₂ is dominantin a solution phase and HCO₃ ⁻ is dominant in a solid phase, if pH of asolution is maintained within 3.5<pH<5.3.

Heretofore, H. C. Urey et al (The Journal of Chemical Physics, Vol. 11,No. 9, September, 1943, pages 403-412) have already measured theequilibrium constant for isotope exchange K between HCO₃ ⁻ and CO₂ andit is 1.013 to 1.014 at 25° C. ##STR4## Accordingly, C¹³ can beconcentrated into a solid phase in the present method.

Next, procedures for conducting the present method will be illustratedbelow. Procedures may be divided roughly into a batchwise operation anda chromatography operation.

In every operation, a preferable separation efficiency can be attainedby keeping a reaction temperature at 0° C. to 150° C. Where carbonicacid is selected as a carbon-containing acid, a particularly preferableseparation efficiency can be attained at a reaction temperature of 20°C. to 90° C. Moreover, in case of carbonic acid being used as acarbon-containing acid, a separation efficiency can be much moreimproved because of increase in the dissolved CO₂ amount in a solutionphase and consequent increase in a collision frequency between CO₂ andHCO₃ ⁻, if reaction systems of a resin phase and a solution phase areplaced under a pressure ranging from 1 kg/cm² --gauge to 30 kg/cm²--gauge.

A method for the separation of the isotopes of carbon by said batchwiseoperation is to dissolve a carbon-containing acid or its salt such assodium salt, potassium salt, ammonium salt, lithium salt and the like inwater, add an anion exchange resin thereto, adjust pH of a solutionphase to a value as defined in the above equation (4), more preferablythe above equation (5), with an acid such as hydrochloric acid, sulfuricacid and the like or an alkali such as sodium hydroxide, potassiumhydroxide and the like and stir thoroughly, whereby an equilibrium forisotope exchange is reached. Thereafter, a solid phase and a solutionphase are separated by means of a filter.

Generally, in case that K in the above equation (2) is more than 1, C¹³is concentrated into a solid phase and then the chemical speciesadsorbed as an acid-dissociated type is released as a non-dissociatedtype by the supply of an acid such as hydrochloric acid or sulfuric acidand recovered. On the other hand, in case that K in the above equation(2) is less than 1, C¹³ is concentrated into a solution phase and thensaid solution phase portion is recovered.

Of the operations for practicing the present method in which anacid-dissociated type chemical species of a carbon-containing acid and anon-dissociated type chemical species of said acid are employed incombination with an anion exchange resin, the most advantageous methodconsists in multiplying isotopic exchange reactions between a solidphase and a solution phase by a chromatography operation as explainedbelow. This is to utilize an interconversion between saidacid-dissociated type chemical species and said non-dissociated typechemical species, depending upon pH of a solution. In other words, thisis to utilize the fact that said acid-dissociated type chemical speciesbecomes dominant at a higher pH range than pKa value of the acidemployed, while said non-dissociated type chemical species becomesdominant at a lower pH range.

For example, an anion exchange resin is packed into a column and saidanion exchange resin is converted to its OH⁻ adsorption form by thesupply of an aqueous solution of such alkali as sodium hydroxide orpotassium hydroxide (a OH⁻ concentration is about 0.01 to about 0.9 M/l)to a resin bed in 10 to 20 times amount of a packed volume of saidresin. Then, an aqueous solution of a non-dissociated type of acarbon-containing acid is fed into the column from the upper portionthereof. At this point, said non-dissociated type chemical species isadsorbed on an anion exchange resin as an acid-dissociated type chemicalspecies converted by the reaction with the OH⁻ adsorbed on said resinbed. Successively, an aqueous solution of an acid such as sulfuric acidor hydrochloric acid, which possesses a H⁺ concentration to show asufficiently lower pH than pKa of the carbon-containing acid to beapplied, is fed thereinto. At this point, the absorbed acid-dissociatedtype chemical species is again converted to the non-dissociated typechemical species, which is then released from the anion exchange resin.The so released non-dissociated type chemical species flows downwardsthrough a solution phase to reach a OH⁻ adsorbed band wherein saidspecies is then again converted to the acid-dissociated type chemicalspecies and adsorbed.

Thus, an acid-dissociated type chemical species is present in a solidphase and a non-dissociated type chemical species is in a solutionphase, within the band on which the acid-dissociated type chemicalspecies is adsorbed, and then a continuous contact of both chemicalspecies can be accomplished by the supply of an acid.

As explained above, the separation of the isotopes of carbon can beextremely efficiently effected by the multiplication of isotopicexchange reactions between solid-solution phases using a chromatographyoperation.

BEST MODE OF CARRYING OUT THE INVENTION

This invention is illustrated by way of examples for more detaileddescription.

REFERENCE EXAMPLE 1

100 cc of a 0.03 M/l NaHCO₃ aqueous solution were admixed with 10 g (drybasis) of a strongly basic resin produced by the reaction of a basepolymer, chloromethylated polystyrene beads using divinyl benzene as across linking agent, with trimethylamine and stirring was made for 5minutes while maintaining at 25° C.

After HCO₃ ⁻ was brought to an adsorption equilibrium, the pH of thesolution was adjusted to various values by the dropwise addition of H₂SO₄. Stirring was continued over 30 minutes so that an isotopic exchangeequilibrium was gained and subsequently a resin phase was separated froma solution phase by means of a filter.

The HCO₃ ⁻ adsorbed on the resin was released in the form of CO₂ bywashing with 0.1 M/l sulfuric acid and collected as a solution in 0.1M/l sodium hydroxide solution.

A ratio of the carbon isotopes in the resin and the solution wasmeasured by means of an electron-bombardment type mass spectrometer(manufactured by Shimadzu Corp., Japan: Type GCM5 7000) and K' values asshown below were determined.

    ______________________________________                                        pH    .sup.--C.sup.13 /.sup.--C.sup.12 (× 10.sup.2)                                          C.sup.13 /C.sup.12 (× 10.sup.2)                                                      K'                                          ______________________________________                                        2.0   1.166           1.1654       1.0005                                     2.5   1.171          1.165        1.005                                       3.0   1.170          1.163        1.006                                       3.5   1.172          1.158        1.012                                       4.0   1.173          1.158        1.013                                       4.5   1.172          1.158        1.012                                       5.0   1.172          1.159        1.011                                       5.3   1.170          1.158        1.010                                       5.5   1.170          1.162        1.007                                       6.0   1.169          1.163        1.005                                       6.3   1.170          1.164        1.005                                       7.0   1.168          1.165        1.003                                       ______________________________________                                    

It can be seen from the above presented K' values that a practicallypreferred isotope exchange equilibrium constant between a solid and asolution can be obtained by the adjustment of pH to, preferably, 2.5 to6.3 and, more preferably, 3.5 to 5.3.

In this invention, _(p) H value was measured by means of a digital _(p)H meter Type 701A manufactured by Orion Co., Ltd., U.S.A.

REFERENCE EXAMPLE 2

Into a 10--liter four-necked flask equipped with a stirrer and athermometer were introduced 3000 g of water, 20 g of sodium polyacrylateand 82 g of sodium chloride as a suspending agent, 900 g of styrene, 35g of ethyl vinyl benzene, 65 g of divinylbenzene, and as polymerizationsolvents, 380 g of methyl benzoate, 320 g of isoamyl alcohol, 1100 g ofn-heptane and 14 g of azobisisobutyronitrile and thorough stirring waseffected to disperse oil droplets. This mixture was subjected topolymerization at 70° C. over 28 hours. After completion of thepolymerization, it was cooled, a resin was placed into a washing towerwith filters and then washed well with 10 liters of methanol and a largeexcess amount of water.

After washing, the resin was dried at 40° C. under 2 mmHg for 72 hours,and 300 g of the resin thus dried were introduced into 3 liters ofchloromethyl methyl ether while maintained at 5° C. Chloromethylationwas conducted by reaction using 450 g of zinc chloride as a catalystover 48 hours and, successively, amination was accomplished in a 20%ethanolic solution of trimethylamine at 40° C. over 5 hours to afford astrongly basic anion exchange resin (_(p) Ka_(R) >14).

REFERENCE EXAMPLE 3

Into a 5--liter four-necked flask equipped with a stirrer and athermometer were placed 3000 g of water and 4 g of sodium polyacrylateand 12 g of sodium chloride as a suspending agent were dissolvedthereinto and stirred. Then, 250 g of styrene having dissolved therein 2g of benzoyl peroxide, 20 g of ethyl vinyl benzene, 30 g ofdivinylbenzene and, as a polymerization solvent, 138 g of triol wereadded thereto and thorough stirring was effected to disperse oildroplets.

Polymerization was carried out at 70° C. over 28 hours. After completionof the polymerization, it was cooled and placed into a washing towerwith filters, through which 20 liters of methanol and a large excessamount of water were passed to thoroughly wash the thereby producedresin. After washing, the resin was dried by means of a vacuum drier at40° C. under 2 mmHg for 16 hours.

To 2 liters of chloromethyl methyl ether while maintained at 5° C. wereadded 120 g of the resin thus dried. Chloromethylation was effected byreaction using 250 g of zinc chloride as a catalyst over 48 hours and,successively, amination was done in a 15% ethanolic solution ofdiethylamine at 70° C. over 30 minutes to afford an anion exchange resin(_(p) Ka_(R) =8.0 at 25° C.).

EXAMPLE 1

Into a Pyrex glass cylindrical column with an internal diameter of 8 mmand a height of 1000 mm was packed the strongly basic ion exchange resin(_(p) Ka_(R) >14) prepared in Reference Example 2 to form a resin bedwith a height of 900 mm. As the carbon-containing acid, there wasemployed carbonic acid (_(p) Ka=6.3 at 25° C.). Approximately 1000 cc ofa 0.1 M/l NaOH aqueous solution at 25° C. were fed thereinto, therebythe anion exchange resin was converted to its OH⁻ adsorption form.Subsequently, 300 cc of an aqueous solution of CO₂ saturated anddissolved under a pressure of 1 kg/cm² --gauge were supplied thereto.The dissolved CO₂ was converted to HCO₃ ⁻ by reaction with the adsorbedOH⁻ and in turn adsorbed on the resin to form HCO₃ ⁻ adsorption bands.

Thereafter, an aqueous solution of 0.03 M/l H₂ SO₄ was fed into thecolumn at a flow rate of 4.5 cc/min. at the upper portion thereof. Theadsorbed HCO₃ ⁻ was released in the form of CO₂, which flew downwardsthrough a solution phase to contact with the OH⁻ adsorption band and itwas reconverted to HCO₃ ⁻ and re-adsorbed thereon.

A HCO₃ ⁻ adsorption band portion descended with a continuous supply ofH₂ SO₄, whereby C¹³ was gradually concentrated at the upper portion ofthe HCO₃ ⁻ adsorption band.

After the HCO₃ ⁻ adsorption band reached the lowest end of the column,CO₂ as eluted was collected as its salt form in 0.1 M/l NaOH atintervals of 5 minutes and then a ratio of carbon isotopes was measuredby means of the above-mentioned electron-bombardment type massspectrometer.

Elapsed time (Developing time) from start of the H₂ SO₄ aqueous solutionsupply and ratio of isotopes in the eluted CO₂ are as shown below.

    ______________________________________                                        Developing time (min.)                                                                         C.sup.13 /C.sup.12 (× 10.sup.2)                        ______________________________________                                        205.0            1.163                                                        210.0            1.167                                                        215.0            1.183                                                        220.0            1.199                                                        225.0            1.256                                                        230.0            1.298                                                        ______________________________________                                    

The pH value as measured in a solution phase of the adsorbed band was3.9.

EXAMPLE 2

Into a Pyrex glass cylindrical column with an internal diameter of 8 mmand a height of 1000 mm was packed the same strongly basic ion exchangeresin as used in the Example 1 to form a resin bed with a height of 900mm. As the carbon-containing acid, there was employed carbonic acid(pKa=6.3 at 25° C.). Approximately 1000 cc of a 0.1 M/l NaOH aqueoussolution at 25° C. were fed thereinto, thereby the anion exchange resinwas converted to its OH⁻ adsorption form. Subsequently, 40 cc of anaqueous solution of CO₂ dissolved under a pressure of 10 kg/cm² --gaugewere supplied thereto, whereby a HCO₃ ⁻ adsorption band was formed.

Thereafter, an aqueous solution of 0.2 M/l H₂ SO₄ was fed into thecolumn at a flow rate of 3.5 cc/min at the upper portion thereof.

After the HCO₃ ⁻ adsorption band reached the lowest end of the column,CO₂ as eluted was collected as its salt form in 0.2 M/l NaOH atintervals of 1 minute and then a ratio of carbon isotopes was measuredby means of the above-mentioned electron-bombardment type massspectrometer.

Developing time and ratio of carbon isotopes as measured are shownbelow.

    ______________________________________                                        Developing time (min.)                                                                         C.sup.13 /C.sup.12 (× 10.sup.2)                        ______________________________________                                        42.5             1.165                                                        43.5             1.183                                                        44.5             1.226                                                        45.5             1.274                                                        46.5             1.361                                                        47.5             1.382                                                        ______________________________________                                    

Further, the pH of a solution phase within HCO₃ ⁻ adsorption band asmeasured at this point was 3.7.

EXAMPLE 3

Into a Pyrex glass cylindrical column with an internal diameter of 8 mmand a height of 1000 mm was packed the anion exchange resin prepared inthe Reference Example 3 (pKa=8.0 at 25° C.) to form a resin bed with aheight of 900 mm.

As the carbon-containing acid, there was employed carbonic acid (_(p)Ka=6.3 at 25° C.).

Approximately 1000 cc of a 0.1 M/l NaOH aqueous solution at 25° C. werefed thereinto, thereby the anion exchange resin was converted to its OH⁻adsorption form. Subsequently, 35 cc of an aqueous solution of CO₂dissolved under a pressure of 12 kg/cm² --gauge were supplied thereto,whereby a HCO₃ ⁻ adsorption band was formed. Thereafter, a 0.2 M/l H₂SO₄ aqueous solution was fed into the column at a flow rate of 4.0cc/min at the upper portion thereof.

After the HCO₃ ⁻ adsorption band reached the lowest end of the column,CO₂ as eluted was collected as its salt form in 0.2 M/l NaOH atintervals of 1 minute and then a ratio of carbon isotopes was measuredby means of the above-mentioned electron-bombardment type massspectrometer.

Developing time and ratio of carbon isotopes as measured are shownbelow.

    ______________________________________                                        Developing time (min.)                                                                         C.sup.13 /C.sup.12 (× 10.sup.2)                        ______________________________________                                        34.0             1.165                                                        35.0             1.173                                                        36.0             1.207                                                        37.0             1.249                                                        38.0             1.333                                                        39.0             1.393                                                        ______________________________________                                    

Further, the pH of a solution phase with with HCO₃ ⁻ adsorption band asmeasured was 4.0.

PROBABILITY OF UTILIZATION IN INDUSTRY

As stated hereinabove, the method for the separation of carbon isotopesthrough a carbon isotope exchange reaction between a solid phase and asolution phase according to this invention is extremely efficient andcapable of inexpensively separating and concentrating C¹³, as comparedwith separation methods between a gas phase and a liquid phase, betweena gas phase and a solution phase or between a solution phase and asolution phase according to the chemical exchange method in the priorart. Therefore, the future utility and value of the present invention isvery high since the need for C¹³ has increased more as a tracer in avariety of fields, such as clinical medicine, pharmaceutics,biochemistry, agriculture or the like.

What is claimed is:
 1. A method for the separation of the isotopes ofcarbon, C¹² and C¹³, comprising effecting an isotope exchange reactionwith respect to carbon between a dissociated type chemical species of acarbon-containing acid and a non-dissociated type chemical species ofsaid acid, wherein an aqueous solution of a dissociated type chemicalspecies of a carbon-containing acid and a non-dissociated type chemicalspecies of said acid are contacted with an anion exchange resin as asolid phase to adsorb the dissociated type chemical species of the anionexchange resin, while allowing an isotope exchange reaction with respectto carbon to proceed between the dissociated type chemical species andthe non-dissociated type chemical species, thereby separating C¹² andC¹³, respectively, into a solution phase and the solid phase if anequilibrium constant K for an isotope exchange reaction represented by##STR5## wherein K represents an equilibrium constant for isotopeexchange, CXH represents a carbon-containing acid and CX⁻ represents anacid-dissociated type chemical species of the carbon-containing acidismore than 1 or separate C¹² and C¹³, respectively, into the solid phaseand a solution phase if K as defined above is less than 1, whereby C¹³is concentrated in the phase into which C¹³ is separated.
 2. A methodfor the separation of the isotopes of carbon according to claim 1,characterized in that pKa of the above-mentioned carbon-containing aciddefined as --log Ka when an acid dissociation constant of saidcarbon-containing acid is defined as Ka and pH of said solution phasesatisfy the following relation, ##EQU3##
 3. A method for the separationof the isotopes of carbon according to claim 1 or 2, characterized inthat pKa_(R) of the above-mentioned anion exchange resin defined as--log Ka_(R) when an acid dissociation constant of said anion exchangeresin is defined as Ka_(R) and pH of said solution phase satisfy thefollowing relation, ##EQU4##
 4. A method for the separation of theisotopes of carbon according to claim 2, characterized in that pKa ofthe carbon-containing acid as defined above and pH of said solutionphase satisfy the following relation, ##EQU5##
 5. A method for theseparation of the isotopes of carbon according to claim 1, characterizedin that carbon isotope exchange reactions between solid-solution phasesare multiplied by a chromatography operation which comprises repeatingthe steps of effecting contact of an aqueous solution containing anacid-dissociated type chemical species of a carbon-containing acid and anon-dissociated type chemical species of said acid with said anionexchange resin by supplying a solution containing a non-dissociated typechemical species to a bed of an anion exchange resin in a OH⁻ adsorbedform packed into a column from the upper portion thereof, thereby toform an adsorbed band of said acid-dissociated type chemical species onsaid resin; and of subsequently releasing the acid-dissociated typechemical species adsorbed on said resin as the non-dissociated typechemical species again by supplying an acid to said anion exchange resinbed from the upper portion thereof.
 6. A method for the separation ofthe isotopes of carbon according to claim 1 wherein saidcarbon-containing acid is carbonic acid.
 7. A method for the separationof the isotopes of carbon according to claim 6, characterized in thatsaid contact of an aqueous solution containing an acid-dissociated typechemical species of a carbon-containing acid and a non-dissociated typechemical species of said acid with an anion exchange resin is effectedunder a pressure of 1 kg/cm² --gauge to 30 kg/cm² --gauge.
 8. A methodfor the separation of the isotopes of carbon according to claim 1,characterized in that said contact of an aqueous solution containing anacid-dissociated type chemical species of a carbon-containing acid and anon-dissociated type of said acid with an anion exchange resin iseffected at 0° C. to 150° C.
 9. A method for the separation of theisotopes of carbon according to claim 6 or 7, characterized in that saidcontact of an aqueous solution containing an acid-dissociated typechemical species of a carbon-containing acid and a non-dissociated typechemical species of said acid with an anion exchange resin is effectedat 20° C. to 90° C.